It wasn’t until the late afternoon, when the air took on a yellow color and began to smell like filthy sewers, that the people of London grew concerned.

Hoping the odd fog would clear by morning, they went to sleep.

The next day, the air was green and a horrendous stench violated their noses.

Soon, what people believed to be fog grew too thick to see through and breathing it in stung their lungs.

For five days the fog rested, assaulting the people of London and murdering thousands.

When the smog cleared on December 9th, it left behind 12,000 dead citizens and over 150,000 hospitalized victims.

The incident came to be known as London’s Great Smog.

It was immediately blamed on emissions from coal burning, but the science behind the smog remained a mystery – until now.

According to phys.org, an international team of scientists, including Texas A&M University-affiliated researchers, believe they know exactly what happened.

Renyi Zhang, a researcher from Texas A&M, University Distinguished Professor, and Harold J. Haynes Chair of Atmospheric Sciences and Professor of Chemistry, joined forces with graduate students and researchers from China, Florida, California, Israel and the UK to discover the truth.

Their findings were published in the current issue of “Proceedings of the National Academy of Sciences” (PNAS).

By experimenting in China, which is home to some of the world’s worst air quality and polluted cities, Zhang and his team were able to discover what exactly caused the deadly smog in London.

They held laboratory experiments and took atmospheric measurements in China to discover sulfate and sulfuric acid particles derived from sulfur dioxide released by coal burning was to blame.

“People have known that sulfate was a big contributor to the fog, and sulfuric acid particles were formed from sulfur dioxide released by coal burning for residential use and power plants, and other means,” Zhang explained.

The team was unsure how the sulfur dioxide was turning into sulfuric acid but they quickly discovered it had to do with nitrogen dioxide, which is a co-product of coal burning.

Unfortunately for London, by burning coal regularly, natural fog diluted the acids and evaporated into smaller acidic haze particles, leading to illness and death.

Zhang goes into depth, saying: “But how sulfur dioxide was turned into sulfuric acid was unclear. Our results showed that this process was facilitated by nitrogen dioxide, another co-product of coal burning, and occurred initially on natural fog.

“Another key aspect in the conversion of sulfur dioxide to sulfate is that it produces acidic particles, which subsequently inhibits this process.

“Natural fog contained larger particles of several tens of micrometers in size, and the acid formed was sufficiently diluted. Evaporation of those fog particles then left smaller acidic haze particles that covered the city.

Unfortunately for China, similar air chemistry is happening in some of its more polluted cities.

In fact, of the twenty most polluted cities in the world, China hosts 16 of them.

Zhang stated: “The difference in China is that the haze starts from much smaller nanoparticles, and the sulfate formation process is only possible with ammonia to neutralize the particles.

“In China, sulfur dioxide is mainly emitted by power plants, nitrogen dioxide is from power plants and automobiles, and ammonia comes from fertilizer use and automobiles.

“Again, the right chemical processes have to interplay for the deadly haze to occur in China. Interestingly, while the London fog was highly acidic, contemporary Chinese haze is basically neutral.

After battling air pollution for decades, Zhang explained China has been attempting to lessen the air quality issue.

The air is so toxic, people have to wear breathing masks and the past two decades have seen an industrial and manufacturing boom, resulting in larger factories and an increase of pollutants.

Zhang explained the only way China can recover is if scientists can better understand air chemistry.

“The government has pledged to do all it can to reduce emissions going forward,” he stated, “but it will take time.